It is known that the light efficacy of a fluorescent lamp and the stability of the light flow improve when the fluorescent lamp is operated at a higher frequency in comparison with a line frequency of 50 Hz to 60 Hz.
Conventional electromagnetic ballasts having an electromagnetic transformer and a starting device do not meet modern requirements of a high light efficacy, a low harmonic distortion and an extended life time of a fluorescent lamp, because the ballast operates at a low frequency and a voltage spike are not controlled when starting. Recently, an electronic ballast is introduced to meet these requirements.
The basic unit of an electronic ballast operating at a high frequency of 10 kHz to 80 kHz consists of a rectifier, a high-frequency converter and a resonant inductor-capacitor (L-C) circuit, where a discharge fluorescent lamp is included into the resonant L-C circuit.
A good electronic ballast should take care of the following characteristics to secure a long service life of a fluorescent lamp as well as the ballast: (1) cathode filaments of the fluorescent lamp should be normally preheated with an exception of an instant start fluorescent lamp; (2) the voltage spike should be kept low for a soft start; (3) the discharge current should be stabilized after turn-on of a fluorescent lamp; (4) the fluctuation of an input voltage should be considered for stability of the charge and light flow of a fluorescent lamp; (5) the power factor and end of life behavior of a fluorescent lamp should be considered; (6) the harmonic distortion of a fluorescent lamp should be low; and (7) the electromagnetic interference should be avoided.
EP Patent No. 0359245 discloses an electronic ballast but it does not meet all the above requirements. The majority of modern fluorescent lamps are supplied with cathodes filaments located at the ends of a glass tube to improve the starting behavior and thus to extend the life time of the lamps. Preheating of the cathode filaments creates space charges which reduce an ionization voltage significantly and thus facilitate a soft start of a lamp--a start of arranged movement of ions and an avalanche increase of electrical current in a lamp. In order to extend the life time of a fluorescent lamp, the preheating current should be regulated properly and the starting voltage minimized to protect emitters from a strong starting current spike and voltage spike.
In an electronic ballast of DE Patent No. 3835533 A1, a thermally sensitive resistor, a thermistor is added in parallel to a starting capacitor to regulate preheating current. The thermally sensitive resistor provides a means of preheating of cathodes before starting a fluorescent lamp. With the main switch on, a large preheating current begins to flow through the cathode filaments due to a low resistance value of a thermistor. When the resistance of the thermistor switches to a high resistance state with the preheating current flowing through it, the main resonant circuit starts working and a high voltage develops across the lamp enough for starting the lamp. Because the resistance of a thermistor depends on the surrounding temperature, the preheating current cannot be accurately regulated at a wide range of operating temperature. In addition, when a lamp is turned off and turned on again in a short time, the preheating effect will be diminished due to a slower recovery of resistance of a thermally sensitive resistor.
GB Patent No. 2267002 discloes an electronic ballast based on a resonant L-C circuit in which an auxiliary capacitor is connected in parallel to a fluorescent lamp to preheat filaments in addition to a starting capacitor. The auxiliary capacitor is disconnected in a predetermined time by a timer relay switch and then the main resonant process provides a high voltage for a start-up of a lamp. In this prior art, the preheating current for cathode filaments is supplied by the charging and discharging process of the capacitor, which shows a big in-rush preheating current that is deleterious to the life time of a lamp.
U.S. Pat. No. 5,319,284 discloes an electronic ballast consisted of a rectifier, a pulse generator connected with a half-bridge transistor switch converter, a first resonant L-C circuit with a damping circuit connected to a fluorescent lamp, and a second resonant circuit with the inductor and capacitor connected in parallel to the lamp. In this prior art, the second resonant circuit having a higher resonating frequency provides a means of preheating of cathode filaments while the first resonant circuit having a lower resonating frequency provides a starting voltage. In this prior art, it is very difficult to set the preheating and starting conditions precisely due to dual resonating circuits with inductors and capacitors.